cpupower: Add AMD P-State sysfs definition and access helper

Introduce the marco definitions and access helper function for
AMD P-State sysfs interfaces such as each performance goals and frequency
levels in amd helper file. They will be used to read the sysfs attribute
from AMD P-State cpufreq driver for cpupower utilities.

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>